Wiring of the Motor Self-locking Circuit

The following are the wiring methods and key points of the motor self-locking control circuit, which are applicable to the basic control of three-phase asynchronous motors:

 I. Required Components

1. **Circuit Breaker (QF)**: The main power switch, providing overload/short-circuit protection.

2. **Contactor (KM)**:

   - Main contacts: Control the power supply of the motor (3 pairs of normally open contacts).

   - Auxiliary contacts: 1 pair of normally open contacts for self-locking.

3. **Thermal Relay (FR)**: Protects the motor from overload (the normally closed contact is connected in series in the control circuit).

4. **Start Button (SB2)**: A normally open button that controls the energization of the contactor coil.

5. **Stop Button (SB1)**: A normally closed button that cuts off the power supply of the contactor coil.

6. **Motor (M)**: A three-phase asynchronous motor.

 II. Wiring of the Main Circuit

1. **Power Inlet**:

   - The three-phase power sources L1, L2, and L3 are connected to the upper terminals of the circuit breaker QF.

2. **Circuit Breaker → Contactor**:

   - The outgoing wires from the lower terminals of QF are connected to the incoming terminals (L1, L2, L3) of the main contacts of the contactor KM.

3. **Contactor → Thermal Relay**:

   - The outgoing terminals (T1, T2, T3) of the main contacts of KM are connected to the incoming terminals of the heating elements of the thermal relay FR.

4. **Thermal Relay → Motor**:

   - The outgoing terminals of FR are connected to the three-phase windings (U, V, W) of the motor M.

 III. Wiring of the Control Circuit

1. **Power Extraction**:

   - A single-phase power supply (AC 220V) is led out from the auxiliary contacts (such as L1, N) of the circuit breaker QF.

   - Or directly extract power from L1 and L2 of the main circuit (AC 380V, selected according to the voltage of the contactor coil).

2. **Stop Button → Start Button → Contactor Coil**:

   - L1 → SB1 (normally closed) → SB2 (normally open) → KM coil → N.

3. **Realization of Self-locking**:

   - Connect the auxiliary normally open contact of the contactor KM in parallel across both ends of SB2 (that is, lead a wire from one end of SB2 to the other end of the KM coil).

4. **Protection of the Thermal Relay**:

   - Connect the normally closed contact of FR in series in the control circuit (for example, connect it before or after SB1).

 IV. Wiring Schematic Diagram (Described in Words)

Main circuit:  

 V. Action Principle

1. **Starting**:

   - Press SB2, the KM coil is energized → the main contacts are closed → the motor runs.

   - At the same time, the auxiliary normally open contact of KM is closed, and the coil remains energized after releasing SB2 (self-locking).

2. **Stopping**:

   - Press SB1, the KM coil is de-energized → all contacts are reset → the motor stops.

3. **Overload Protection**:

   - When the motor is overloaded, the heating element of FR acts → the normally closed contact of FR is disconnected → the KM coil is de-energized.

### VI. Precautions

1. **Voltage Matching**:

   - The voltage of the contactor coil (such as AC 220V or AC 380V) must be consistent with the control power supply.

2. **Wiring Specifications**:

   - Select the cross-sectional area of the wires in the main circuit according to the power of the motor (for example, use 2.5mm² copper wires for a 4kW motor).

   - Use 1mm² copper wires for the control circuit, and the terminal numbers should be clear.

3. **Safe Operation**:

   - Disconnect the power supply before wiring and use a multimeter to check the continuity of the circuit.

   - When powering on for the first time, first test whether the control circuit operates normally, and then connect the motor.

4. **Troubleshooting**:

   - If the motor does not start: Check the power supply, whether FR is reset, and whether the KM coil is burned out.

   - If self-locking cannot be achieved: Check whether the auxiliary contact of KM has poor contact.

### VII. Extended Functions

- **Multi-location Control**: Connect multiple start buttons in parallel and stop buttons in series.

- **Forward and Reverse Rotation Control**: Add another contactor and interlock contacts to prevent the power supply from short-circuiting.

If you need specific component models or circuit diagrams, you can supplement the motor power and control requirements!